There are numerous ways to measure pressure by optical means: Fabry Perot and Fibre-Bragg gratings technologies, Raman Scattering, Brillouin scattering, membrane integrated waveguides, mechanical resonant oscillators, evanescent field coupling or photonic crystals. The technology includes use of birefringence induced by inhomogeneous stress distributions for the measurement of pressure.
The use of birefringence for estimating stress distribution is called “photoelasticity”. It was developed at the beginning of last century and is well established. For example, thin plates of glass or plastic are exposed to external forces and the stress distribution inside the plates can be determined from the fringe pattern created by transmitted polarized light with help of a perpendicular polarization filter. Details of this method can be found, e.g., in the textbook of Föppl and Mönch (L. Föppl, E. Mönch, Praktische Spannungsoptik, Springer, Berlin 1972), the disclosure of which is hereby incorporated by reference in its entirety.
The use of the birefringence effect in structured fibres with fibre core for measuring pressure was known at least in mid 1980. More detailed studies have followed more than a decade later. The geometry and temperature dependence of the effect, stability with respect to environmental influences and ageing and detection setups have been investigated.
The temperature sensitivity of inherent birefringence is at least an order of magnitude greater than that of pressure dependent birefringence, and so there can be considerable advantage in minimizing an inherent temperature component or contribution to birefringence. If two lengths of fiber are made identical and the fibres are rotated 90° relative to one another about their longitudinal axis at the joint or splice, as taught by Dakin and Wade (J. P. Dakin and C. Wade, “Compensated polarimetric sensor using polarisation-maintaining fibre in a differential configuration”, Electron. Lett., Vol. 20, No. 1, pp. 51-53, 1984), then the inherent birefringence in the two parts is identical, and if the two parts are subject to the same pressure conditions also the pressure dependent birefringence cancels. So, if the two pieces of fibre core with a cladding are identically sensitive to pressure, a useful signal will be obtained if the two pieces of fibre experience different pressures. This is the approach taught by Dakin and Wade. By this kind of setup, one may minimise the temperature dependence of the measured pressure-induced phase shift.
There is an alternative approach in which a useful signal can be obtained when the two pieces of fibre experience the same pressure from the exterior. If the pressure dependent birefringence is made not equal in the two parts and yet the inherent birefringence is the same in the two parts, the pressure in the two parts may be equal, that is the two pieces of fibre may experience the same pressure, and yet temperature compensation of the inherent birefringence still occurs. This is the basis for the concept as disclosed in U.S. Pat. No. 5,515,459, where the pressure induced birefringence is made to differ by admitting the external pressure selectively to some of the side-holes formed in the fibre.
Reference is also made to GB 2 419 401 A, in which a fiber with a light guiding core and a cladding is used for differential pressure measurement, and wherein one single pressure is applied to the device in both sensing parts in order to allow overall differential measurement with biasing portion optically coupled in series with the two sensing parts.
The disclosures of all of the foregoing documents are incorporated by reference in their entireties.